1. Field of the Invention
The present invention relates to an electrically powered linear actuator providing reciprocating, linear motion in applications with precise position and force requirements. More specifically it relates to an electrically powered linear actuator and certain methodology inherently taught thereby that addresses applications requiring high forces at moderate to low speeds to perform a specific operation, and high speeds at low forces at other portions of the cycle to quickly position and remove tooling. Typical applications include piercing, crimping, pressing, trimming, forming, flaring, clamping, bending, coining, marking, and riveting.
2. Description of Prior Art
Permanent magnet motors are typically limited in speed by the back electro-motive force (bemf) generated with respect to the available driving voltage. For this reason cycle times are often compromised due to less than desirable approach speeds. Additionally, extremely high currents are often required to deliver sufficient force, increasing both the cost and size of the motor windings and drive. It has been noted that many approaches have been taken to overcome speed limitations in permanent magnet motors through flux control or field weakening.
The most common approach to field weakening requires the drive to introduce an imposing field against the fixed excitation from the magnets. This is often achieved by injecting a non torque producing current to counteract the magnet flux. This approach is cumbersome and requires relatively high waste currents to be injected into a device where efficiency is important and heat dissipation is often a challenge. Another approach to field weakening of permanent magnet devices is through hybrid motor designs. Some of these include double salient, consequent pole, and a number of axial flux variations. There is a nearly endless variety of hybrid permanent magnet motor designs that exist to overcome the weakness of permanent magnet motors. Very few of these are in production due to various limitations, primarily in cost to manufacture.
A third approach to field weakening is by mechanical design. The primary approaches for mechanical field weakening include the use of an actuator to shift the rotor relative to the stator, and the use of centrifugal force to expand an external rotor with permanent magnets under high speed, increasing the radial air gap. The complexity of these devices is primarily due to the desire to overcome the limitations in permanent magnet motors for purely rotary applications. Much of this development is in an attempt to achieve constant horsepower at high speeds for electric or hybrid vehicles. Thus, the devices are usually intended only to supply torque with improved high speed performance. The approach of axially shifting the rotor relative to the stator requires a linear device to provide the motion. This adds significant cost and complexity to a rotary motor.
In the current invention the primary intent is to provide a device that provides linear motion rather than rotary motion. This device will utilize a traditional screw mechanism for converting rotary to linear motion. The device will also contain an integrated permanent magnet, brushless motor technology widely used today. The screw/nut will become an integral part of the rotor and rotor assembly, capable of axial motion in addition to the linear motion provided external to the device. This transition takes place automatically based on the load applied.
Without the ability to field weaken in applications requiring high forces/low speeds and high speeds/low forces motors and drives are larger than necessary, operate far from peak efficiency for the application, and are more costly than necessary. It is generally most desirable to minimize the current requirement from the drive necessary to generate the required force. The application throughput will dictate cycle time requirements, and hence linear travel motion profiles. Maximum speed requirements in combination with the screw lead and available supply voltages will establish the maximum motor voltage constant (Ke), and hence, the maximum motor torque constant (Kt).
Force requirements along with the established Kt will define current requirements of the motor and drive. Tradeoffs can be made between motor diameter, length, magnet type, number of winding turns and pattern, and generally the overall size and shape of the motor. In applications where high force/low speed and high speed/low force are required, operation does not take advantage of the peak horsepower capability of the typical permanent magnet motor. The back electromotive force (bemf) produced by the permanent magnets limit how high the Ke can go for a given available voltage. By limiting the Ke, the Kt is also limited. The lower that the Kt must be, the higher the current must be to produce a given force. At high forces these motors can be quite large to accommodate large diameter wire required in the windings. Motor costs, as well as drive costs, are typically high to supply the necessary currents.
What is needed, therefore, is a structural device or system that functions to limit the magnetic flux produced by the magnets at the time high speeds are required. This would allow the Ke, and hence, the Kt to increase and reduce current requirements, reducing component size and cost, decreasing energy consumption, and relieving the devices thermal dissipation requirements. In the current permanent magnet servo actuator this can be done through field weakening by allowing the rotor to shift at various positions within the stator.
It is desirable for the device to automatically shift between the high force and high speed states on demand. The high force or high speed should be able to be achieved at any point along the actuator for any length of stroke. In addition, many of the applications for this device include the need for force feedback for quality control purposes. By monitoring the position and force a detailed indication of the process/component quality is provided. The need for this force monitoring is often beyond that achieved through observing the applied current, since friction and many other system inefficiencies and tolerances influence the precision that can be obtained from purely monitoring current. Therefore, it is further desirable to integrate into the device a low cost, precise means of measuring the applied force without the need for a high cost force sensor and signal conditioner.
It is further noted that applications requiring linear motion for high force applications have traditionally used hydraulics or air over oil devices. While these devices provide high speeds and high forces their limitations are in that they are only capable of very crude positional, speed, and force control, they are very bulky systems, and due to the use of hydraulic fluid they can be very messy over time as seals wear and begin to leak.
In order to overcome these limitations electrically powered devices are replacing hydraulics in many applications. A wide variety of electrically powered linear actuators exist in prior art and in application dating back to the 1960's. These devices typically include a screw and nut to convert rotary motion from a motor to linear motion. The motor may be coupled to the end of the screw or built directly onto an extension of the screw shaft. Other configurations include motors that allow the screw to pass through the center of the motor rotor to shorten the overall package.
More recent patents and application publications set forth claims in which the screw nut or force application shaft are allowed to traverse axially within the rotor. For example, U.S. Pat. No. 6,223,971 discloses a nut and force application member that are smaller in diameter than the motor rotor such that the inner diameter of the rotor is used as a guide for the force application member. The screw is rotated with the nut held rotationally fixed. Another variation of this device is presented in U.S. Pat. No. 6,603,228 in which a brake is further disclosed. U.S. Patent Application Publication No. 2005/0253469 discloses structure for allowing the nut to pass axially within the rotor, the nut being rotationally fixed. The current invention does not intend to have a nut that passes within the rotor. The nut is allowed to rotate with the rotor, and hence the screw moves axially.
Many of these devices include the use of permanent magnet, brushless motors. These motors are the best suited motors for precision, linear reciprocating devices due to their high torque density, high efficiency, rapid dynamic response for precise position, velocity and force control, and limited maintenance. Permanent magnet, brushless motors have existed since the 1950's with many specific variations of these since that time. The intent of the current device is to utilize the most commonly available state of the art rotor and stator permanent magnet, brushless components, not otherwise made the subject of enforceable patent claims.
The prior art, however, perceives a need to overcome limitations that exist with linear devices that incorporate permanent magnet, brushless motors. Permanent magnet, brushless motors are typically limited in speed due to the back electromotive force (bemf) produced by the magnets. Reduction of this speed limitation or field weakening has been presented in numerous forms, however always presented with respect to a device with rotary output rather than linear. One particular approach to field weakening is by axially shifting the rotor within the stator. This particular approach is shown, for example, in U.S. Pat. Nos. 7,042,128; 6,943,478; 6,555,941; and 6,492,753. All of the noted devices produce a rotary output and require a linear actuator to shift the rotor.
The prior art thus perceives a need for a linear device having an automatically shifting rotor within a rotor-stator assembly for producing a wider range of torque speed operation. The current invention is a linear device that through driving of the linear device (i.e. by means of a screw/nut, into a substantial load change) the rotor is forced to automatically shift within the rotor-stator assembly, producing a wider range of torque speed operation than previously accomplished in devices with the rotor fixed axially with respect to the stator.